Double-quantum Nuclear Magnetic Resonance Research Articles

Influence of network structure determined by Time-domain [formula omitted] Double Quantum NMR on the creep properties of non-stoichiometric epoxy-amine resins aimed for chemical anchoring applications

Chemical anchors, used to fasten structural components, are prone to creep under sustained loading, which significantly affects their long-term performance and service life. In this study, the molar ratio, that determines the ultimate network structure of an epoxy-amine resin system, is systematically modified to investigate the creep properties. The percentage of physical entanglement and defects (free and dangling chains) for each formulation is obtained by Time-Domain 1H Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) measurements. The crosslink density values from the dynamic mechanical analysis (DMA) and the network parameter from the NMR measurements show a strong linear correlation. Tensile tests were performed to determine the influence on the mechanical properties and set the load levels for tensile creep measurements of the selected formulations. Epoxy-rich systems (Epoxy-Amine adduct) show the highest modulus and tensile strength but inferior creep properties compared to stoichiometric and amine-rich (Amine-Epoxy adduct) compositions. This can be attributed to the build-up of several network structures with higher content of free and dangling chains leading to lower deformation resistance. At high loads, the entanglement density has an increasing impact on the creep behavior compared to defect-rich systems. Modeling using the Findley approach was carried out to link the obtained creep parameter with the material properties. Therefore, this work aims to deepen the understanding of the molecular-scale structures of epoxy resins required for material and design optimization, by investigating the correlation between network structure and creep properties.

Effects of artificial weathering in NR/SBR elastomer blends

Degradation of polymer blends occurs by the constituent phases undergoing distinct chemical changes that depend on their unique chemical structures. This makes predicting and establishing a structure-property relationship for each phase necessary as well as challenging. In this work, the molecular and physical changes occurring in sulfur-cross-linked natural rubber (NR), styrene−butadiene rubber (SBR), and their 50/50 blend subjected to accelerated weathering are analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, atomic-force microscopy (AFM), and dynamic mechanical thermal analysis (DMTA). NMR transverse relaxation time (T2) studies suggest the formation of rigid components due to weathering. FTIR and AFM reveal that this is related to the formation of a stiff surface due to chemical modifications, which shows up as an additional thermal transition in the DMTA curves. Low-field double-quantum (DQ) NMR studies of the cross-link density, by the residual dipolar coupling constant (Dres), of SBR show a continuous increase in its cross-link density over the weathering duration (988 h). In contrast, NR exhibits dominant chain scission reactions resulting in defects, with both materials demonstrating the formation of different chain lengths. During the first 168 h, NR also undergoes modification of sulfur bond lengths, which is also observed in the blend. The blend largely follows an intermediate trend of cross-link densities compared to the two polymers but shows signs of lesser chain modifications than a weighted average of the two polymers. This is confirmed by phase-resolved DQ magic-angle spinning (MAS) NMR experiments whereby the peak-specific Dres of the blend was measured to be less modified than that of the individual vulcanizates, thus proving that the blend is more resistant to weathering than its constituent elastomers.

ACCELERATED HEAT AGING OF BUTYL AND BROMOBUTYL AIRCRAFT INNER TUBES FOR SHELF LIFE DETERMINATION

ABSTRACT Butyl and bromobutyl inner tubes, specified by the Aerospace Standard AS50141 for military aircraft, were thermally aged from 40 to 120 °C for varying lengths of time and then their hardness and mechanical properties were measured. 1H double quantum nuclear magnetic resonance (DQ NMR) was used to elucidate crosslink density and distribution changes. Time–temperature superposition of the aged data coupled with the Arrhenius approach was used to determine an approximate shelf life. High (80–120 °C) and low (40–80 °C) temperature oxidation processes were occurring for both rubbers. Below 80 °C, an increase in crosslink density, hardening, stiffening, and loss of elongation was observed. Plasticizer and volatile loss contributes to compound stiffening. Sulfur crosslink network modifications during thermal aging can explain ultimate property loss and stiffness increase. Diffusion limited oxidation was taking place above 80 °C, with the development of a thin oxidized layer composed of ionic crosslinking that affected both hardness and mechanical properties. For butyl rubber, the hardness rise stabilizes as do the ultimate properties, likely due to the proliferation of chain scission reactions, whereas crosslinking reactions prevailed over chain scission events for bromobutyl rubber. Crosslink density and defect fractions B and C as measured through DQ NMR were in agreement with the physical property testing results. The degree of heterogeneity of the network as perceived visually through DQ NMR regularization increases upon exposure to higher temperatures and longer aging times due to the broadening of the crosslink density distribution. Similar Arrhenius activation energies were calculated for the low and high temperature oxidation process for butyl and bromobutyl rubbers. The projected shelf life for the butyl and bromobutyl inner tubes was 10 and 20 yr, respectively. For the first time, DQ NMR testing results (crosslink density and its distribution, defect level) have been successfully applied to support a shelf life determination.

Nonprestretching double-network enabled by physical interaction-induced aggregation

The design of double-network (DN) improves the properties of matrix effectively, which has attracted wide attention. The recently reported construction of DN structure usually involves the prestretching process (swelling process). However, such complicated multistep swelling processes become more difficult in high molecular weight matrix, which are not suitable for constructing DN structure in the universal elastomers (for example, natural rubber and polyisoprene rubber). To expand the application fields of DN, this work designs the dense crosslinking domain and the sparse crosslinking domain to construct nonprestretching DN structure through physical interaction-induced aggregation. We find that physical interaction induced-aggregation form the dense crosslinking domain and the sparse crosslinking domain, which builds the two contrasting DN structures. Double-quantum nuclear magnetic resonance, dynamic mechanical analyzer, and mechanical property characterization demonstrate that the dense crosslinking domain as the brittle first network is preferentially ruptured upon deformation, leading to a large energy dissipation. This design concept represents a general approach to develop the nonprestretching DN structure, which expands the application fields of DN structure in the elastomer matrix.

A valid and efficient mechanical performance degradation assessment method for cushion member material EPDM based on DQ NMR inspection and static compression test

The aim of this study is to establish a valid and efficient inspection method for assessing macroscopic mechanical property degradation of ethylene-propylene-diene-monomer (EPDM) by examining crosslinked density variation of microstructure. Inspection methods of mechanical behavior and crosslinked density are based on static compression and double quantum nuclear magnetic resonance ( DQ NMR), respectively. After introducing the hyperelastic constitutive model proposed by Yuhai Xiang and designing different conditions of heat-aging and fatigue load, the significant mechanical parameter variation is observed for the hyperelastic constitutive model. The significant impacted factors include heat-aging time and fatigue load except for heat-aging compression strain. The phenomenon is consistent with DQ NMR test results. We further analyze relationship between macro and micro properties of EPDM. After importing fitting assumption and limitation of DQ NMR data to control fitting error, the fractions of dangling and sol chains and residual dipolar couplings constants in DQ NMR parameters show moderate negative () and high positive () correlation with of constitutive model parameters, respectively. Thus the validity and precision of inspection method is proven to estimate behavior variation of EPDM and hopefully the method can be extended to analyze other rubber-like materials.

Modified Li7P3S11 Glass-Ceramic Electrolyte and Its Characterization.

Li7P3S11 glass ceramics have high conductivities competitive with liquid electrolytes, making them good candidates as solid-state electrolytes for all-solid-state lithium-ion batteries. However, the metastable nature and performance of Li7P3S11 glass ceramics remain mysterious. Herein, modified Li7P3S11 glass ceramics with compositions of 70Li2S-30P2S5 were prepared via two-step mechanical milling and thermal annealing. Li7P3S11 glass ceramics synthesized using the conventional method (mechanical milling and thermal annealing) were again ball-milled to obtain amorphous 70Li2S-30P2S5 with a peculiar glass structure. Further thermal annealing was carried out to crystallize the glass. The obtained crystalline phase was analogous to the original Li7P3S11 phase, but the conductivity was enhanced by a factor of 1.7. Based on 31P solid-state nuclear magnetic resonance (NMR) spectroscopy, the Li7P3S11 phase contained an additional PS43- unit. A rational deconvolution procedure for the 31P solid-state NMR spectra based on crystalline Li7P3S11 was developed and applied to the samples. The analysis can resolve the additional crystalline PS43- unit in the Li7P3S11 structure. Based on two-dimensional double-quantum 31P NMR spectroscopy, the additional PS43- unit is located adjacent to the P2S74- unit, suggesting that P2S74- is divided into two PS43- units in the Li7P3S11 phase. The flip motion of Li+ was also investigated based on the 7Li spin-lattice relaxation time. The independent activation energy of spin-lattice relaxation with respect to temperature in the Li7P3S11 phase was attributed to a conduction path between the two PS43- units. The findings provide a synthetic route that can be used to develop metastable solid-state electrolytes.

Spectroscopic Signatures of MQ-Resins in Silicone Elastomers

Polysiloxane elastomers have a large application space due to their versatile cross-linking chemistry and highly tunable physical and mechanical properties. One approach for improving the mechanical integrity of commercial polysiloxane “silicone” elastomers while maintaining their optical transparency is the addition of small, silicone-resin molecules to the network. However, both the poly(dimethylsiloxane) (PDMS) network and the silicone-resin particles have an amorphous structure and complex chemistry, which makes the characterization of their structural properties and segmental network dynamics difficult. Here, we report the synthesis and characterization of a series of model silicone networks modified with a specific class of silicone-resin known as MQ-resin using Raman and advanced nuclear magnetic resonance (NMR) spectroscopy methods. Raman spectroscopy was successfully used to quantify the contribution of the MQ-resin to the network, to determine the type of MQ-resin present in the network, and to investigate the completeness of the network cross-linking reaction. Solid-state and 1H double-quantum (DQ) NMR spectroscopies were used not only as a detection method for the MQ-resins but also to quantify changes in the segmental dynamics of the network as a function of MQ-resin concentration. The combination of Raman and NMR spectroscopies describes a series of samples where the MQ-resin particles and PDMS chains maintain their independent segmental dynamics up to high concentrations of MQ-resins (40–50% MQ), where the physical properties of the resin dominate the physical properties of the overall network. The results from our spectroscopic analyses are consistent with the results from macroscopic characterization techniques such as solvent uptake and mechanical testing. The spectroscopic insights into the structure–property relationships of PDMS-MQ composites presented in this study are a valuable tool not only for the synthesis and reverse engineering of future generations of commercial silicone elastomers but also for understanding the mechanisms of aging and degradation over the material lifetime.

HEAT AGING OF A BROMOBUTYL TIRE INNER LINER UNDER AEROBIC AND ANAEROBIC CONDITIONS

ABSTRACT A bromobutyl tire inner liner compound was prepared and subjected to aerobic and anaerobic heat aging at a temperature of 100 °C for seven aging times up to 8 weeks. Hardness and mechanical properties were monitored, and the evolution of the crosslink density was followed using equilibrium solvent swell and low field double quantum (DQ) nuclear magnetic resonance (NMR). The hardness and the 300% tensile stress increased with heat aging, while both tensile strength and elongation at break dropped. Both chain scission and crosslinking reactions were taking place. Equilibrium swelling and DQ NMR results confirmed that a larger crosslink density increase was seen under aerobic versus anaerobic aging conditions. The network distribution consisting of a dominant low crosslinking zone and small areas of higher crosslinking slowly broadened and shifted toward higher crosslink densities upon heat aging. The compounds aged heterogeneously. Attenuated total reflectance–Fourier transform infrared spectroscopy confirmed the presence of an oxidized surface layer, and therefore diffusion-limited oxidation effects, but only under aerobic aging conditions. Reaction mechanisms are proposed to explain the net crosslink rise with heat aging.

Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol).

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.

Structure and Connectivity in an Amorphous Silicon Oxycarbide Polymer-Derived Ceramic: Results from 2D 29Si NMR Spectroscopy

The atomic structure of the Si–O–C tetrahedral network of an amorphous silicon oxycarbide polymer-derived ceramic (PDC) of the composition SiO0.94±0.11C1.13±0.08 was studied at both the short range and the intermediate range using 1D and 2D 29Si nuclear magnetic resonance (NMR) spectroscopic techniques, respectively . The 1D 29Si magic angle-spinning NMR spectrum of the PDC indicates that the Si–O–C network consists of SiO4, SiO3C, SiO2C2, and SiC4 units with relative abundances of approximately 26, 25, 20, and 29%, respectively. The 2D 29Si extended CSA amplification spectrum of this PDC shows that the chemical shift anisotropy (Δ) of the mixed-bond SiOxC4–x units is significantly higher than that of the SiO4 units. On the other hand, the unusually high Δ-value for the SiC4 units was interpreted to be indicative of its role as the connecting element between the Si–O–C network and the free-carbon nanodomains. The 2D 29Si double-quantum correlation NMR spectrum of this PDC indicates that there is extensive direct linking between SiO4 and SiO3C units in the Si–O–C network besides the connectivity between like SiOxC4–x units, while the SiO4 and SiO2C2 units are only linked via a SiO3C unit. In contrast, the SiO3C units show no restriction in linking with the other SiOxC4–x units in the network. Finally, the SiC4 units show significant clustering, which is consistent with their spatial localization at the interface between the Si–O–C network and the sp2 C nanodomains. Such a spatial distribution of the SiOxC4–x units is argued to be consistent with their mass-fractal dimensions measured in previous studies.

Dynamic heterogeneity in homogeneous polymer melts.

Chain entanglement behaviors were studied by 1H Hahn echo nuclear magnetic resonance (NMR) and 1H double-quantum (DQ) NMR experiments. Poly(ethylene oxide) (PEO) was chosen to investigate the chain entanglement behaviors. The 1H Hahn echo NMR results demonstrate that the critical molecular weight of PEO is approximately 6 kg mol-1. Above this critical molecular weight, chain entanglements start to occur in the melts resulting in anisotropic motions of polymer chain. The 1H DQ NMR observations establish that PEO melts with molecular weights above the critical value exhibit dynamical entanglements. The entangled networks, formed by PEO with a molecular weight of 480 kg mol-1 (PEO480), present slow mobility and rather homogeneously distributed chain entanglements, while the entangled networks, formed by PEO with a molecular weight of 255 kg mol-1 (PEO255), present fast mobility and obvious dynamic heterogeneity in the distribution of chain entanglement. Short chain PEOs like that with a molecular weight of 2 kg mol-1 are demonstrated to function like solvents when being added in an appropriate concentration to PEO480, and the dilution effect increases the chain mobility of PEO480. Moreover, properly diluted PEO480 networks exhibit dynamic heterogeneity similar to that observed in PEO255.

Study on Homogeneity in Sulfur Cross-Linked Network Structures of Isoprene Rubber by TD-NMR and AFM – Zinc Stearate System

An important role of the zinc stearate (ZnSt2) activator in the sulfur cross-linking reaction of isoprene rubber is proved by combination of 1H double-quantum nuclear magnetic resonance spectroscop...

The sulfur reversion process in natural rubber in terms of crosslink density and crosslink density distribution

Unfilled natural rubber compounds composed of conventional (CV), semi-efficient (SEV), efficient (EV) and sulfur donor (SD) vulcanization systems were heat aged to promote sulfur reversion. Rheometry, hardness, strain-strain characteristics including Mooney-Rivlin analysis, equilibrium solvent swell and Double Quantum (DQ) Nuclear Magnetic Resonance (NMR) were used to monitor crosslink density changes. A loss of crosslink density was observed by rheometry, C1, equilibrium swelling and by DQ NMR as a function of cure extent. No chain scission reactions were operating in the time/temperature conditions used. All crosslink distributions were unimodal and the network homogeneity followed the order of EV > SD > SEV > CV. The crosslink distribution narrowed during the curing process for the CV and SEV systems. Non-oxidative maturation reactions were advantageous in promoting a more random distribution of crosslinks in the polymer matrix.

CHARACTERIZATION AND CORRELATION OF THE NETWORK CHAIN DENSITY TO THE PROPERTIES OF FLUOROELASTOMER RUBBER

ABSTRACTThe concentrations of triallyl isocyanurate (TAIC) in a peroxide-curable fluoroelastomer terpolymer containing 67 wt% of fluorine were varied to generate compounds of differing crosslink densities. Experimental analysis was undertaken using rheometry, hardness, stress–strain (Mooney–Rivlin), equilibrium solvent swell, and low-field nuclear magnetic resonance (NMR) using the double quantum (DQ) technique. Increasing the TAIC concentration caused a systematic rise in rheometry elastic torque, hardness, and tensile strength, whereas both elongation at break and swelling levels decreased. These results are concurrent with an enhanced overall level of crosslinking, which was confirmed by the steady increase of the Mooney–Rivlin C1 values. DQ NMR analysis using hydrogen and fluorine probes and subsequent application of fast Tikhonov regularization to the corrected intensity data were particularly useful in discerning the inhomogeneous nature of the compound morphology. The spatial distribution of the crosslink density suggests that the compound consists of small, highly crosslinked/entangled polymerized TAIC domains embedded within the elastic crosslinked matrix. A concentration of 3 phr of TAIC is optimal according to compression set testing.

Solid-state nuclear magnetic resonance investigation of synthetic phlogopite and lepidolite samples.

In the present work, our aim is to decipher the cationic ordering in the octahedral and tetrahedral sheets of two Al-rich synthetic materials, namely, phlogopites of nominal composition K(Mg3-x Alx )[Al1+x Si3-x O10 ](OH)y F2-y and lepidolites in the system trilithionite-polylithionite with composition K (Lix Al3-x )[Al4-2x Si2x O10 ](OH)y F2-y , by directly probing the aluminium distribution through 27 Al and 17 O magic-angle spinning, multiple-quantum magic-angle spinning, and 27 Al-27 Al double-quantum single-quantum nuclear magnetic resonance (NMR) experiments. Notably, 27 Al-27 Al double-quantum single-quantum magic-angle spinning NMR spectra, recorded at 9.34 and/or 20.00 T, show the spatial proximity or avoidance of the Al species inside or between the sheets. In both studied minerals, the ensemble of NMR data suggests a preference for [4] Al in the tetrahedral sheet to occupy position close to the [6] Al of the octahedral sheets.

Aging of natural rubber studied via Fourier-transform rheology and double quantum NMR to correlate local chain dynamics with macroscopic mechanical response

Nowadays mainly the linear rheological properties of natural rubber (NR) vulcanizate during aging are studied, even though NR is rheologically nonlinear under most application conditions. Here, the nonlinearity was quantified via Fourier transform rheology (FT-Rheology) using I3/1, the relative intensity of the 3rd harmonic. The crosslink density and the content of defects were investigated by the 1H double quantum nuclear magnetic resonance (DQ-NMR). For aerobic aging at 120 °C, the crosslink density increases, but later turns into a broader mesh size distribution with more defects. Accordingly, the rheological nonlinearity of the material increases in the earlier stage and then decreases. For mechanical aging, the nonlinearity continuously decreases as a function of the fatigue cycle numbers, whereas the local crosslink density of the polymer network remains constant. This behavior can be assigned to the micro-cracks generated along adjacent network defects, which are larger than the detection scale of DQ-NMR. Compared to the linear rheological parameters, storage modulus G′ and loss factor tanδ, I3/1 exhibits higher sensitivity to the aging of crosslinked NR under both high temperature aerobic and mechanical fatigue conditions.

EXPERIMENTAL DETERMINATION OF THE QUANTITY AND DISTRIBUTION OF CHEMICAL CROSSLINKS IN UNAGED AND AGED NATURAL RUBBER. II: A SULFUR DONOR SYSTEM

ABSTRACTA series of unfilled and stabilized natural rubber compounds varying in concentration of tetramethylthiuram disulfide (TMTD) was analyzed using rheometry, hardness, dynamic mechanical properties, stress–strain (Mooney–Rivlin), equilibrium solvent swell (Flory–Rhener), and low-field nuclear magnetic resonance (NMR) by the double quantum (DQ) technique. Crosslinking level increased proportionately with TMTD concentration, and the reaction ratio of three TMTD molecules producing one crosslink was generally upheld. Unreacted TMTD acted as a pseudo-plasticizer and lowered the chain entanglement density with increasing TMTD content. DQ NMR confirmed that the elastic network was homogeneous and that the absolute chemical crosslink distributions broaden with increasing curative level. Upon mild heat aging, zinc complexes based on TMTD/ZnO are likely responsible for causing additional crosslinking, explaining the rise in crosslink density by equilibrium solvent swell and DQ NMR. The amine-based antioxidant, the generation of thiocarbamate radicals from TMTD, and the heat stability of the predominant monosulfide crosslinking system helped to limit network breakdown through chain scission. The chain entanglement increase is likely due to reduction of the plasticizing effect caused by unreacted curative. The distribution of crosslinks slightly broadens toward higher total crosslink density because of the generation of additional chemical crosslinks and chain entanglement densification.

Nonideality in Silicone Network Formation via Solvent Swelling and 1H Double-Quantum NMR

The versatile cross-linking chemistry of poly(dimethylsiloxane) (PDMS)-based materials affords a large research space in which polymers with widely varying elastomeric properties may be synthesized. Parameters such as chain length, cross-link density, cross-link functionality, filler content, and chain chemistry can all be modified to produce materials with specific physical and mechanical properties. Commercial polysiloxane-based, “silicone” elastomers are generally intractable, which makes the precise characterization of their networks problematic. We report here the application of equilibrium solvent uptake analysis and 1H double-quantum nuclear magnetic resonance (1H DQ NMR) spectroscopy to determine the network topology of end-linked PDMS networks with nonideal network topology. Despite their structural complexity, we can quantify both the classical and nonclassical contributions to network structure using 1H DQ NMR which are in reasonable agreement with solvent uptake data. These findings serve as the foundation for future investigations of even more complex commercial silicones using 1H DQ NMR.

Research on architecture and composition of natural network in natural rubber

Though the superior properties of natural rubber (NR) have been attributed to its special network architecture, the currently accepted model “naturally occurring network” is far from describing its authentic network structure. In this paper, we focused on the composition of the chain entanglements in the network structure of unvulcanized NR. By using synchrotron wide-angle X-ray diffraction (WAXD), the evolution of its strain-induced crystallization (SIC) behaviors was real-time traced, and the stress-strain behaviors at various strain rates and temperatures were also tested. The results demonstrated that the entanglements can act as crosslinking points to increase the network chain density and lead to the easier SIC behavior. By applying the tube model to analyze the stress-strain curves of unvulcanized NR, we found that the contribution of the entanglement network to the stress is about an order of magnitude larger than that of the network formed by two terminal groups. Via double-quantum nuclear magnetic resonance (1H-DQ NMR), we further detected two types of entanglement in NR and deproteinized NR (DPNR), i.e. transiently trapped entanglements (TTEs) and permanently trapped entanglements (PTEs), among which TTEs are the main composition of the entanglement network. Moreover, the network formed by two terminals causes more PTEs in NR, while due to lacking the constraint of proteins, more TTEs exist in DPNR. Based on the research, we proposed a novel network model for unvulcanized NR.

Contribution of Entanglements to Polymer Network Elasticity

Trapped entanglements, cross-linker functionality, and elastically effective chains are the sources of elasticity of polymer networks and gels. However, despite more than 80 years of theoretical and experimental research in this field, still little is known about their relative contribution to network elasticity. In this work, we use double quantum nuclear magnetic resonance (DQ NMR) experiments to characterize the elasticity of model polymer networks prepared with cross-linkers of mixed functionality and control of structural defects. An order parameter that condensates the elastic response within the theoretical framework of the entangled phantom theory for rubber elasticity was identified. Standard lore dictates that low molecular weight precursors for the elastically active chains leads to a negligible contribution of trapped entanglements. Here we show that the contribution of trapped entanglements may equal the contribution coming from elastically active material and that it is independent of networ...